Respiratory disease remains the primary cause of morbidity and mortality worldwide due to infectious disease such as community acquired respiratory tract infections (RTIs). Beta-lactam or macrolide antibiotics are currently, a first line of treatment for most RTIs, however, a considerable number of pathogens are developing resistance to these current treatments. A primary resistant mechanism of bacteria towards macrolide/ketolide antibiotics is through target modification (rRNA methylation) whereby 5-O-sugar substituent (desosamine) plays a critical role. Under Phase I, unprecedented macrolide medicinal chemistry was successfully developed that enabled the synthesis of novel 5-O-sugar modified ketolide derivatives that displayed excellent activity against macrolide-resistant bacteria. The specific aims of this Phase II proposal are to continue our progress toward identifying a preclinical drug candidate through a series of chemistry lead optimization experiments. The continued medicinal chemistry efforts to improve in vitro and in vivo antibacterial activity will not only be guided by in-house minimum inhibitory concentration (MIC ) determinations (against primary and secondary panels) and in vivo oral efficacy experiments using relevant disease infection models, but in addition, will be guided by computer simulated ligand-receptor docking experiments utilizing computer aided molecular design (CAMD), thus allowing for rational based glycodesigri of our antibacterial analogues. The lead compounds will be further evaluated by obtaining important MIC50/90 data against relevant pathogens (Streptococcus, Haemophilus, and Staphylococcus) and by determining important animal-plasma and -lung pharmacokinetic parameters. Phase II funding will also support studies to determine resistance development and the potential for cross-resistance between our lead compounds and other leading macrolides/ketolides, an important criteria for assessing a compound's future marketability. Finally, preliminary safety will be assessed of our most promising leads by performing mammalian cell cytotoxicity assays and by determining acute oral toxicity in rodents.